The Legend of Zelda: Skyward Sword Wiki Guide

Tentalus

Tentalus is the Boss of the Lanayaru Sandship. Upon opening the boss door you enter a room with a column in the center. You walk in and massive tentacles erupt from the floorboards. Fi advises you to return to the top of the ship and as you do so the boat begins to fill with water. You reach the top and Tentalus erupts from the water and the fight begins.

Tentalus will shoot tentacles through the floor boards in order to try and grab you. Don't stand in one place too long. Run around while lifting your sword in the air to charge your Skyward Strike. Once it's full, lock on to the nearest tentacle and let a horizontal strike rip. This will sever the tentacle(s) in its path. Should a tentacle grab you, shake your controllers to escape.

After you cut a few tentacles, Tentalus will pop its head out of the water and look at you. Shoot its eye with an arrow to bring it down (be sure to dodge the incoming tentacle attacks if you miss your shot), then run in close to attack with your sword. The moment you hit the eye with the arrow, charge forward. The falling tentacles don't hurt you.

Note: Do not use the Bow to attack the eye after Tentalus is stunned. You may trigger a glitch(the game refuses to switch to Phase II if you attack exclusively with the Bow in Phase I)that will force you to reset your Wii.

After you've done this two or three times, the battle moves on to the second stage. Tentalus wrecks part of the platform. Turn around and head for the fallen crate, dodging barrels that are in your parth. Climb up and face Tentalus again. Its tentacles now take the form of snakes. Just stand in the middle of the platform and swipe at them as they attack. No need to dodge. Once you've cut a few of them, you can again draw your bow and send an arrow into Tentalus's eye. Run in close and finish the job!